1. Field of the Invention
This invention relates to a stencil duplicating machine, and more particularly to stencil duplicating machine which includes a mechanism for contacting and detaching a press roller with and from a printing drum, and a mechanism for actuating a sheet separating pawl to separate a printed sheet from the printing drum.
2. Description of the Related Art
There is a conventional stencil duplicating machine which uses a mechanism for contacting and detaching a press roller with and from a printing drum without interfering with a stencil clamp (called "clamp" hereinafter) mounted on the printing drum. Such a mechanism includes a pair of cams which are fitted on a shaft separate from a shaft for the printing drum, and actuates the press roller in synchronization with the printing drum.
Japanese Utility Model Laid-Open Publication No. Sho 59-59361 exemplifies an improved mechanism for contacting and detaching a press roller with and from a printing drum. Specifically, the mechanism is integral with the printing drum and includes a pair of cam discs for the press roller which are coaxially attached to the printing drum. The phase of these cam discs is determined in such a manner that the press roller is in contact with the printing drum while the press roller is in contact with base circles of these cam discs via a pair of rollers as cam followers, and that the press roller is out of contact from the printing drum while it is in contact with large diameter portions of the cam discs via these rollers. During the contact with the base circles of the cam discs, the press roller is not interfered with the clamp.
Japanese Patent Laid-Open Publication No. Sho 61-262,125, for example, discloses a mechanism for preventing a sheet separating pawl from being interfered with a clamp in a stencil duplicating machine. During a non-sheet separating process, this mechanism allows the sheet separating pawl to stay at a position centrifugally away from the surface of a printing drum without interfering with the clamp. Such a position is called "second non-sheet separating position" (a first non-sheet separating position will be described later).
In this mechanism, a cam which is disposed on one side of the printing drum and also serves as a flange comes into contact with a cam follower which is disposed at one end of an arm for actuating the sheet separating pawl, thereby operating the sheet separating pawl. Further, the mechanism includes a lever, which is used to adjust a space between the surface of the printing drum and a tip of the sheet separating pawl, and is coaxial with the sheet separating pawl. This mechanism enables the sheet separating pawl to strip a printed sheet from the surface of the printing drum without contacting therewith even when the printing drum is not completely circular or even when the printing drum vibrates. In other words, at the sheet separating position, the sheet separating pawl always maintains a predetermined Space between the printing drum and itself.
Further, there have been proposed various methods of actuating the sheet separating pawl, e.g. a method of actuating the sheet separating pawl by using a cam which is mounted on a shaft independent of the shaft of the printing drum but is operated in synchronization with the printing drum, or a method in which an arm for the sheet separating pawl is actuated at a proper timing.
In the first mentioned mechanism, the press roller is prone to a problem that it does not always contact with the printing drum with a uniform pressure. In the second mentioned mechanism, little consideration is taken to units for pushing and supporting the press roller.
As for the sheet separating pawl actuating mechanism, the first mentioned mechanism is prone to the following problems should it be applied to a small stencil duplicating machine including a slender printing drum only for A4 size sheets. The stencil duplicating machine includes a strip of screen leather adjacent to a clamp on the surface of the printing drum. The screen leather reinforces the trailing edge of a mesh screen of the printing drum, projects approximately 1 to 2 millimeters from the printing drum, and is approximately 40 to 50 millimeters wide. Therefore, it is necessary to determine the size and contact timing of cams for the sheet separating pawl so as to keep the sheet separating pawl at a position where it is not in contact with the screen leather. This position is called "first non-sheet separating position". In the mechanism, the cams and cam followers are required to be in contact with one another over a relatively large area of the printing drum during the rotation of the printing drum. The sheet separating pawl has to quickly return to the sheet separating position via the first and second non-sheet separating positions in succession. A timing to set the sheet separating pawl at the first non-sheet separating position can be easily determined. Conversely, a timing to let the pawl return to the sheet separating position from the second non-sheet separating position is rather difficult since the tip of the separating pawl might collide with the clamp while the sheet separating pawl returns to the sheet separating position. The second timing cannot be set unless the cams are enlarged.
The second mentioned sheet separating mechanism inevitably becomes large since it has a complicated structure to operate the printing drum and associated cam shafts in a synchronized manner. Such a complicated structure requires deliberate and time-consuming adjustment, and would lead to a reduced performance of the stencil duplicating machine.